1. Field of the Invention
This invention relates to the preparation of anhydrous high purity phosphorus pentafluoride in high yield.
2. Description of the Prior Art
Known processes for the production of phosphorus pentafluoride include:
1. A phosphorus pentachloride halogen exchange reaction with arsenic trifluoride, EQU 3PCl.sub.5 +5AsF.sub.3.fwdarw.3PF.sub.5 +5AsCl.sub.3 ; PA1 2. The chlorination of phosphorus trifluoride, EQU 5PF.sub.3 +3Cl.sub.2.fwdarw.3PF.sub.5 +2PCl.sub.3 ; PA1 3. Phosphoryl fluoride reacting with hydrogen fluoride, EQU POF.sub.3 +2HF.fwdarw.PF.sub.5 +H.sub.2 O; PA1 4. Heating a metal fluoride, a phosphoric acid or monofluoro-phosphoric acid and sulfur trioxide; EQU 2H.sub.3 PO.sub.4 +6SO.sub.3 +6 CaF.sub.2.fwdarw.PF.sub.5 +HPF.sub.6.2H.sub.2 O+HF 7+6 CaSO.sub.4 ; PA1 5. Phosphorus pentachloride reaction with hydrogen fluoride, EQU PCl.sub.5 +5HF.fwdarw.PF.sub.5 +5HCl. PA1 a) adding an excess of anhydrous hydrogen fluoride above a stoichiometric amount to polyphosphoric acid to effect reaction in a preferably dry atmosphere to form hexafluorophosphoric acid in the unreacted excess of hydrogen fluoride; and then PA1 b) reacting the hexafluorophosphoric acid with fuming sulfuric acid while in contact with the unreacted excess of hydrogen fluoride to form substantially pure phosphorus pentafluoride; and PA1 c) recovering phosphorus pentafluoride, hydrofluoric acid and sulfuric acid.
The first three processes are described in U.S. Pat. No. 3,584,999 to Wiesboeck. The halogen exchange and the chlorofluorination processes require extensive fractionation to separate mixed halides (PClF.sub.4, PCl.sub.2 F.sub.3), HCl, inter alia from the desired end product, phosphorus pentafluoride. The last process is disclosed in U.S. Pat. No. 3,634,034 to Nickerson and Wiesboeck, which produces along with phosphorus pentafluoride, hexafluorophosphoric acid and calcium sulfate. Separation problems and disposal of large amounts of calcium sulfate are problems which must be addressed with this process. Finally, the reaction involving phosphorus pentachloride and hydrogen fluoride (equation 5) also requires special techniques of separation for undesirable by-products.
Therefore, there exists a need to provide high purity phosphorus pentafluoride in a simple and economical procedure without the need for extensive fractionation or high pressure equipment and not to have extensive acid gases present which require special disposal.